Soils containing PCBs have posed serious environmental hazards. The PCBs are toxic to both plants and animals, are known to bioaccumulate and present serious health hazards. It has been observed that PCBs have extremely slow biodegradation rates and their persistence and toxicity have prompted serious governmental action restricting their use and application. Even trace amounts of PCBs are environmentally undesirable.
Among the halogenated compounds which pose dangers to the environment are halogenated aliphatic compounds, e.g., trichloroethylene, and chlorinated aromatic compounds, e.g., polychlorinated biphenyls (PCBs). Prior to 1970, PCBs were used in a wide range of applications due to their attractive physical properties. They were used as plasticizers, waterproofing compounds, lubricants, heat transfer fluids, hydraulic fluids, etc. Their primary use was in the electrical industry as dielectric media in transformers and capacitors.
A number of methods have been proposed for treating soils containing halogenated organic compounds. Incineration, while an approved method, is expensive and hazardous if uncontrolled. Other methods are based on the use of large amounts of expensive reagents or the treatment of the soil to thermally desorb the PCBs or to solvent-extract the PCBs and subsequently dehalogenate them.
One such method is that disclosed in Peterson, U.S. Pat. Nos. 4,447,541 and 4,574,013. These patents are directed to processes for the decontamination of soil wherein the soil is contacted with a reagent mixture comprising an alkaline compound and dimethyl sulfoxide. The reagent mixture desorbs halogenated organic compounds from the soil and then reacts with them, but the technique is not satisfactory because the soil, sediment, or sludge is dried before the chemical reaction takes place, the process is inefficient, and most importantly, dimethyl sulfoxide is extremely hazardous to employ because it is capable of introducing toxic contaminants through the skin of workers at the site.
Pytlewski et al., in U.S. Pat. Nos. 4,337,368, 4,400,552, 4,349,380, 4,417,977, 4,430,208, 4,471,143, 4,460,797, 4,602,994, and 4,523,043, disclose advantageous processes for decomposing toxic halogenated organic compounds with a decomposition reagent which is the reaction product of an alkali metal hydroxide and a polyglycol, or the monoalkyl ether thereof. The decomposition reagent of Pytlewski et al. may be formed outside the reaction zone or may be formed within a reaction zone by the reaction of the hydroxide and polyglycol, and, optionally, oxygen. The Pytlewski et al. processes are sometimes referred to by the acronyms NaPEG.RTM. or KPEG.TM..
The methods and reagents described in Pytlewski et al. are advantageous because the reagents are relatively inexpensive and are capable of reacting quickly and efficiently with wet or dry soil, sediment, or sludges to reduce the level of PCBs and other contaminants to environmentally acceptable levels.
The Environmental Protection Agency (EPA) has recently developed a process based upon the technology of Pytlewski et al. known as the "base catalyzed destruction method." This process, which is a variant of the NaPEG.RTM. or KPEG.TM. technology, comprises adding an aqueous solution of polyethylene glycol (PEG) to a contaminated soil containing halogenated organic compounds. An alkali metal hydroxide is then added to the contaminated soil. The water added with the PEG distributes the reagents throughout the soil and acts as a wetting agent. The contaminated soil is then heated at a temperature and for a time sufficient to substantially dehydrate the soil leaving the reagents nevertheless well distributed throughout the soil and concentrated to a very reactive state. The soil is then further heated to a temperature between about 100.degree. and 300.degree. C. for a time sufficient to effect destruction of the halogenated organic compounds. Lastly, an acid is added to the soil in an amount sufficient to neutralize the alkali metal hydroxide so that the soil may be returned to its original environment.
The EPA process, discussed in further detail below in connection with FIG. 1, is highly inefficient. For example, PCBs are vaporized together with water in the reaction zone. After condensation in a light solvent and decantation of solvent-PCBs solution from water, the PCBs are chemically destroyed in a separate reaction system. The capital costs and utility costs of this process are prohibitive.